Funnel flow channel

The funnel flow channel essentially consists of two component parts (i) the drained repose or unconfined surface layer, and (ii) the core flow channel confined by the static bed. Should a hopper with this pattern of discharge be refilled before it has emptied, some of the original material... 

Sometimes mass flow develops in silos which were structurally designed for funnel flow. [4] Even if this doesn t occur, the local pressure peak which develops where a funnel flow channel intersects a silo wall can be devastating. [6]... 

A funnel flow bin typically exhibits a first-in/last-out type of flow sequence. If the material has sufficient cohesive strength, it may bridge over the outlet. Also, if the narrow flow channel empties out, a stable rathole may form. This stable rathole decreases the bin s five or usable capacity, causes materials to cake or spoil, and/or enhances segregation problems. Collapsing ratholes may impose loads on the stmcture that it was not designed to withstand. 

Material-Flow Cbaracteristics Two important definitions of the flow characteristics of a storage vessel are mass flow, which means that all the material in the vessel moves whenever any is withdrawn (Fig. 21-17), and funnel flow, which occurs when only a portion of the material flows (usually in a channel or rathole in the center of the system) when any material is withdrawn (Fig. 21-18). Some typical mass-flow designs are shown in Fig. 21-19. 

Often there are situations in which mass-flow bins cannot be installed for reasons such as space limitations and capacity requirements. Also, sometimes the product to be stored has an FF flow function that lies below the flow factorj, bridging takes place, and unassisted mass flow is not possible. To handle these situations, a number of flow assisters are available, the most desirable of which use a feeder and a short mass-flow hopper to enlarge the flow channel of a funnel-flow bin. The choice of feeder or flow assister should always be made as part of the storage-vessel analysis. The resulting systems are then usually as effective as the mass-flow types. 

Two flow patterns can develop in a bin or hopper funnel flow and mass flow. In funnel flow (Fig. 1), an active flow channel forms above the outlet, which is surrounded by stagnant material. This is a flrst-in, last-out flow sequence. As the level of powder decreases, stagnant powder may slough into the flow channel if the material is sufficiently free flowing. If the powder is cohesive, a stable rathole may remain. 

Many applications use screws with constant pitch to feed material from a slotted opening. The configuration shown in Figure 9a shows a constant pitch and constant diameter causing a preferential flow channel to form at the back (over the first flight) of the screw. This type of flow destroys the mass flow pattern and potentially allows some or all of the problems discussed about funnel flow. 

Bidirectional flow, which occurs at a funnel-shaped channel because of the interaction of EOF and HDF, creates a mixing effect [478,479]. Such a bidirectional flow has also been used to trap particles (see Chapter 8, section 8.1.5). 

A non-uniform feed density is not a primary concern. Since tablet presses operate as volumetric feeders, variation of the feed density into the press feed frame can result in tablet weight variation. A funnel flow bin will typically have a more non-uniform feed density than a mass flow bin, since the blend in the funnel flow bin will be subjected to different consolidation pressures depending upon where in the bin it is discharged from. For instance, the blend located at the bottom of the bin at the hopper walls, which is outside the flow channel, may be more consolidated and have a higher density than the blend within the flow channel. 

However, within these expressions lie the seeds of simple misunderstandings. In some cases of what is called funnel flow , the flow channel looks nothing like a funnel. Its shape may vary from an unpredictable route drawing down from a flat surface in an obscure and unstable manner, as within a fluidized product. Fig. 5.3, to the other extreme where the flow channel has broadened to intersect the container walls underneath the surface of the stored contents. Fig. 5.4. The upper part, shown as region 4, moves in a bed flow maimer, while the lower section, region 2, is distinctly core flow. This latter mode of behaviour may be externally assessed as a form of mass flow, but it most certainly is not. 

Funnel flow This term describes combined system of core flow and drained repose, where no portion of the flow channel is slipping on the container walls, see Fig. 5.2 shown earlier. 

A third flow pattern, expanded flow, is a combination of funnel flow and mass flow (see Figure 2.6). Usually this is achieved by placing a small mass flow hopper below a funnel flow hopper. The mass flow hopper section expands the flow channel from the outlet up to the top cross section of the mass flow hopper. It is important to ensure that this cross-sectional area is sufficiently large so as to avoid ratholing in the funnel flow hopper section. Expanded flow designs are generally considered only when the cylinder diameter exceeds 6 m or so. 

J. Carson, D. Goodwill and K. Bengston. Predicting the shape of flow channels in funnel flow bins and silos. In ACI Spring Convention. Boston, 1991. 

Expanded Flow. Expanded dow uses the best aspects of fuimel dow and mass dow by attaching a mass dow hopper section below one that exhibits fuimel dow. The dow pattern expands sufficiendy at the top of the mass dow hopper to prevent a stable rathole from forming in the funnel dow hopper above it. In this way, the dow channel is expanded, material dow is uniform, and the bin height is limited. 

The channel comprises a stoneware funnel, having a collar through which a powerful stream of water enters from different sides, and a stoneware pipe with a U-bend to form a siphon, sketched in Figs. 141 and 142. The diameter of the pipe should be at least 30 cm. Above the funnel there is a hood connected with the fume ducting system. Nitrocellulose is carried by the water flowing down the channel, and thus transported to the next building. 

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